Permanent magnet electric motors/generators (i.e. machines) are used in various applications including as starters/generators coupled to gas turbine engines for aerospace applications. Most permanent magnet machines are “impedance limited” which means that the windings are configured to have high impedance (inductive reactance) to prevent excessively large AC currents from flowing through windings of such machines in the event of a short circuit failure in circuitry connected to or associated with the operation of such machines.
A potential failure mode includes the possibility of DC current being driven into windings of a machine during operation as a motor and/or as a generator. Such flow of DC current into the windings could significantly reduce the protective impedance limit of the machine by saturating protective soft magnetic components. Consequently, such flow of DC current into the windings could tend to stop (e.g. brake) the machine during operation and potentially cause excessive heating. During operation as a generator, a flow of DC current into the windings could cause excessively large AC currents to be output from the machine into one or more low impedance loads connected to the machine and thereby cause a cascading failure.
Improvement in the fail-safe operation of electric machines is therefore desirable.